1. Field of the Invention
The present invention relates generally to apparatus for collecting solar radiation, and more particularly to such apparatus for collecting solar radiation for use in glazing applications.
2. Statement of the Prior Art
There are a variety of types of solar collectors presently in use today. For low temperature applications, it has been demonstrated that the most efficient and inexpensive type of solar collector is a flat plate collector. Such collectors consist generally of an insulated box, which surrounds a flat, metal absorber plate with fluid channels and is covered by a solar radiation access window made of glass. In such well-known applications, the solar radiation enters the box through the access window, and is absorbed by the fluid contained within the channels, thereafter being transferred out of the box via the channels to a storage system.
Notwithstanding the simplicity of such flat plate collectors, they experience serious thermal losses as the temperature of the absorber plate rises. These thermal losses, as is well-known, are usually due to convective and conductive processes which occur at low temperatures, and by re-radiation losses as the temperature within the collector increases. Various strategies have been developed to counter the effects of such convective, conductive and radiation losses. However, the most useful of these strategies has been the provision of an evacuated space above the collector with an integral low emissivity coating applied to the solar access window. See, for example, U.S. Pat. No. 3,987,780 (Nazik et al.) and U.S. Pat. No. 4,579,107 (Deakin). Nevertheless, none of these strategies have been used for glazing applications in a flat plate collector.
One disadvantage occurring with such evacuated devices which are known in the prior art is that, in order to be used to produce high temperatures, they most typically are used in conjunction with parabolic solar tracking mechanisms. These tracking mechanisms are expensive, and their use with such evacuated devices made much more complex by the necessity to maintain accurate focus of the solar radiation. In other words, the evacuated devices must be placed at the focal point of the parabola and continuously maintained in alignment therewith to optimize transfer of solar radiation.
Known evacuated devices also suffer from the disadvantage that they are tubular in shape and have a very small radius of curvature. As a result, any small error in focus results in a high loss of solar radiation because the incident solar radiation will strike the evacuated tube in a nearly tangential manner.
One strategy which has been recently used by researchers in the United Kingdom and Australia to avoid such disadvantages and still provide an efficient yet inexpensive solar collector is to produce an evacuated glazing which uses two flat panes of glass that are supported by low-heat conducting materials and sealed at their edges. Such strategies, however, require either a small glazing surface or extremely thick panes of glass to withstand the stresses imposed by the vacuum.